A conventionally existing ordinary material provides no selectivity for radio waves of the same frequency in terms of a pulse width. As illustrated in FIG. 1A, the pulse width here signifies the length of a radio wave (pulse) per time unit during which the radio wave is generated. The pulse width also signifies a time length (excitation time) during which the radio wave energy is generated.
The resonance phenomenon of a periodic structure determines electromagnetic field characteristics in “a periodic structure shorter than an artificially designed incident radio wave wavelength” referred to as a metasurface. Appropriately designing the periodic structure can provide unusual electromagnetic field characteristics not found in the natural world.
The metasurface described in non-patent literatures 1 and 2 is structured to use full-wave rectification and periodically place a plurality of conductive materials having a conductive property in a lattice-like structure over a planar dielectric substance. Moreover, as illustrated in FIG. 24, a full-wave rectifier circuit 22 configured as a diode bridge links adjacent conductive materials 11. The full-wave rectifier circuit 22 includes an RC circuit 60 that connects a capacitor and a resistor in parallel. This circuit structure is placed where electric fields concentrate. Therefore, the RC circuit 60 is connected between the conductive materials 11 according to a conventional example.
A metasurface in FIG. 24 according to the conventional example is characterized to absorb a short-pulse radio wave and transmit a long-pulse radio wave even if the radio wave uses the same frequency. The operating principle is described below. The capacitor has an impedance expressed as 1/jωC. In the expression, j denotes the imaginary unit, ω=2πf (where f is a frequency), and C denotes the capacitance.
The capacitor can store the high-frequency energy. However, the energy of low-frequency components fully charges the capacitor. The capacitor cannot store the energy any more. The energy stored in the capacitor is then discharged to the resistor. The short-pulse radio wave can therefore dissipate all the energy before the next radio wave arrives.
An incident wave induces a surface current and includes frequency component f in this example. However, rectifying action of the diode gradually converts the frequency component into a direct-current component.
From these viewpoints, the metasurface absorbs the radio wave with a short pulse width (excitation time or waveform) and transmits the radio wave with a long pulse width. Non-patent literatures 1 and 2 describe characteristics as illustrated in FIG. 25. In FIG. 25, the horizontal axis denotes a pulse width, the vertical axis denotes absorptance, a black square denotes a simulation result, and a white square denotes an experiment result.
The non-patent literatures describe that time constant RCC can control the characteristics. Specifically, varying newly specified time constant RCC can horizontally shift a curve in FIG. 25A to a curve in FIG. 25B. FIG. 25B entirely illustrates simulation results. A white circle uses the capacitance (i.e., time constant) ten times larger than a black square. A white triangle uses the capacitance (i.e., time constant) one tenth of a black square.
The developed waveform selectivity according to the conventional example is capable of changing ranges of pulse width absorbed by time constant. However, a shorter pulse always indicates a higher absorptance and a longer pulse or continuous wave always indicates a lower absorptance.
Therefore, the conventional example includes the following issue. The structure using the capacitor and the resistor cannot transmit a short pulse-width signal and absorb a long pulse. Accordingly, the structure cannot absorb or transmit only a signal with an optionally specified pulse width and transmit or absorb the other signals.